An electric power steering apparatus (EPS) serves as an apparatus which is equipped with a motor control unit to control a motor. The electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of the motor, applies a driving force of the motor being controlled with an electric power supplied from an inverter to a steering shaft or a rack shaft by means of a transmission mechanism including a reduction mechanism.
In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control. A brushless motor, which has an excellent maintenance performance, is generally used as the motor.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, a handle shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through a reduction mechanism (an engagement mechanism of a worm and a worm wheel) 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 having a torsion bar are provided with a torque sensor 10 for detecting a steering torque Ts of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction mechanism 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command (steering assist) based on a steering torque Ts detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for EPS based on a voltage control command value Vref obtained by performing compensation and so on with respect to the current command value.
Besides, a steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle θ from a rotational position sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vs from the CAN 40. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40. The control unit 30 includes a control board mounted electronic parts or the like.
In such the electric power steering apparatus, the control unit 30 mainly comprises a CPU (including an MPU, an MCU and so on), and general functions performed by programs within the CPU are shown in FIG. 2.
As shown in FIG. 2, the steering torque Ts detected by the torque sensor 10 and the vehicle speed Vs detected by the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1, based on the steering torque Ts and the vehicle speed Vs with reference to an assist map, which is a control target value of a current supplied to the motor 20. The calculated current command value Iref1 is inputted into a current limiting section 33 via an adding section 32A, the current command value Irefm that is limited the maximum current in the current limiting section 33, is inputted into a subtracting section 32B. A deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im which is fed-back is calculated in the subtracting section 32B, and the deviation I is inputted into a PI-control section 35 for improving a current characteristic of the steering operation. The voltage control command value Vref that the characteristic is improved in the PI-control section 35, is inputted into a PWM-control section 36, and the motor 20 is PWM-driven through an inverter 37 serving as a driving section. The motor current value Im of the motor 20 is detected by a motor current detector 38 and is fed-back to the subtracting section 32B.
A compensation signal CM from a compensation signal generating section 34 is added at the adding section 32A. A characteristic compensation of the steering system is performed by adding the compensation signal CM, and then a convergence, an inertia property and so on are improved. The compensating section 34 adds a self-aligning torque (SAT) 34-3 with an inertia 34-2 at an adding section 34-4, further adds the result of addition performed at the adding section 34-4 with a convergence 34-1 at an adding section 34-5, and then outputs the result of addition performed at the adding section 34-5 as the compensation signal CM. Any of the convergence 34-1, the inertia 34-2 and the self-aligning torque (SAT) 34-3 uses a motor angular speed as a calculation factor.
In the conventional electric power steering apparatus described above, the torque assist control sets the assist torque based on the torque detected by the torque sensor, and in addition performs a friction compensation and an inertia compensation by means of the angular speed of the motor. Further, the conventional electric power steering apparatus includes various compensators and control functions of a superior (vehicle) requirement in order to satisfy respective functions. The result affected with the various compensators or the like returns to a driver as a torsional torque of the torsion bar. Therefore, the more functions increase, the more control design is complicated, and a high technique is required to tune the feeling. Furthermore, the conventional electric power steering apparatus is a control structure that the feeling easily varies dependent on a secular variation of the friction in the mechanical portion and dispersions of the individual units.
The electric power steering apparatus for resolving the above problems are disclosed as follows. For example, in Japanese Unexamined Patent Application Publication No. 2002-160653 A (Patent Document 1), a target value (a torsion bar torque) is set by calculating a vehicle reaction force and a road-surface information (a friction coefficient), and the control is performed based on the deviation between the target value and the actual torque. Further, in Japanese Unexamined Patent Application Publication No. 2004-203089 A (Patent Document 2), a target value (a torsion bar torque) is set and the control is performed based on the deviation between the target value and the actual torque. Furthermore, in Japanese Unexamined Patent Application Publication No. 2009-57017 A (Patent Document 3), a steering motor is controlled with angle information of an upper-side and a lower-side of the torsion bar.
In the engagement of the worm and the worm wheel which constitutes the conventional reduction mechanism, a teeth rattling sound occurs if a gap (backlash) exists. Further, when the gap is too small, the friction torque increases and the feeling near the neutral position of the handle is deteriorated. Consequently, it is necessary for the backlash control to control the parts precisely and also assemble the parts accurately. To resolve this problem, the mechanism to suppress the backlash by applied a preload pressure using a spring, a rubber or the like and does not raise the friction, is proposed (Japanese Patent No. 4196831 B1 (Patent Document 4)).